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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 12

Timebase Principles

a.      Brilliance. This control enables the operator to adjust the brightness of the display to the most suitable level. Varying the control alters the bias between the control grid and the cathode of the c.r.t. thus altering the intensity of the electron beam.

b.      Focus. This control is set to focus the spot to the smallest possible size, otherwise the display appears blurred and the target indications are not distinct.

Type A Display

We met the type A display in Section 1. This was the first display to be used in radar equipment and is probably the simplest to produce. We shall therefore take this as the first example and see what we require the spot to do. We can then go on to find out how the desired results are obtained.

As shown in Fig 3 the spot moves across the screen to form a horizontal trace, and the reflected pulses from distant targets deflect the spot upwards to give blips on the trace. A range scale is fitted against the face of the c.r.t. so that the operator can read the range of any target by noting the position of the selected blip on the scale.

How the Spot Moves in a Type A Display

In this chapter we are concerned only with the trace. We shall therefore break up the display by removing the range scale and disconnecting the receiver from the indicator so that we are left with only a horizontal trace on the screen (Fig 4).

When each transmitter pulse commences, the spot must be on the left-hand side of the screen at point P. If the maximum radar range is 80 miles the spot must move along the trace at a constant speed to reach point Q exactly 860 uS after it leaves point P. This movement of the spot from P to Q is called the sweep; the time taken to travel from P to Q is determined by the maximum range of the equipment and is known as the sweep time.


 

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